This invention relates in general to FM multiplex (MPLX) demodulators, and more particularly to a method and apparatus for pilot signal cancellation in multiplex demodulators.
In the demodulation of an FM stereophonic signal in the pilot tone system, the 19 KHz pilot signal is extracted from the composite FM signal to be demodulated, and a subcarrier signal of 38 KHz is derived from the pilot signal. This subcarrier signal is employed as a switching signal in the demodulator so that stereophonic signals having right ("R") and left ("L") components are produced.
In a conventional FM multiplex demodulator, as shown in FIG. 1, a composite FM stereo signal is applied to terminal 1A, and opposite phase subcarrier signals are applied to terminals 1B and 1C, whereby "L" and "R" demodulation signals are produced at output terminals 1D and 1E, respectively.
When the pilot signal signal, FIG. 2(a), and the subcarrier signals FIG. 2(b) and FIG. 2(c) are applied to their respective input terminals, pilot signal components FIGS. 2(d) and 2(e), and containing harmonic components appear at the output terminals 1D and 1E, respectively. Accordingly, when the composite signal is demodulated merely by the subcarrier signals, the pilot signal contained in the composite signal remains in the demodulation outputs. These pilot signal components reduce the S/N ratio of the demodulated signal, and it is thus necessary to eliminate such components from the demodulated signal.
In conventional methods of eliminating the pilot signal components, a trap for blocking the pilot signal or a low pass filter is employed. However, these conventional methods adversely affect the high frequency (around 15 KHz) phase, amplitude and transmission characteristics of the demodulator outputs.